The following description is used to describe the background and products, materials and production methods that may comprise specific parts of preferred embodiments in the disclosure of this invention.
a) Laminate Floorings.
The majority of all laminate floors are produced according to a production method generally referred to as Direct Pressed Laminate (DPL). Such laminated floors have a core of 6-12 mm fibreboard, a 0.2 mm thick upper decorative surface layer of laminate and a 0.1-0.2 mm thick lower balancing layer of laminate, plastic, paper or like materials.
The surface layer of a laminate floor is characterized in that the decorative and wear properties are generally obtained with two separate layers of paper, one above the other. The decorative layer is generally a printed paper and the wear layer is a transparent overlay paper, which comprises small aluminium oxide particles.
The decor paper is the most critical of the lamination papers as it gives the visual appearance of the laminate. The decor paper weight is generally in the range of 60-150 g/m2.
The overlay paper is generally thinner with a weight of about 20-50 g/m2 and is made of pure cellulose, which is based on delignified pulp. The overlay paper becomes almost completely transparent after lamination and the appearance of the decor paper is visible. Thicker overlay papers with a considerable amount of aluminium oxide particles may give a high wear resistance. The disadvantage is that they are less transparent and the decorative pattern is covered by a grey layer that disturbs the printed pattern.
Printing of decorative papers is very cost efficient. Rotogravure presses with printing cylinders that may have a width of 3 meters and that may run with a speed of up to 600 m/min are used. The printing cylinders are generally produced by conventional mechanical engraving. Recently digital laser engraving has been introduced which allows faster décor development and provides a better décor quality. Solvent-free inks with organic pigments are often used and excess ink is re-cycled.
The printed decorative paper and the overlay are impregnated with melamine formaldehyde resins, generally referred to as melamine resins, and laminated to a HDF core in large discontinuous or continuous laminate presses where the resin cures under high heat (about 170° C.) and pressure (40-60 bars) and the papers are laminated to the core material. An embossed press plate or steal belt forms the surface structure. Sometimes a structured paper is used as a press matrix. The embossing is in high quality floors made in register with the design. The embossing depth is limited to 0.1-0.2 mm (100-200 micron).
Laminated floors may also be produced with direct printing technology. One advantage is that the pressing operation may be avoided and that no printed papers are needed to provide a decorative surface. Hydro printing inks are used to print the décor by a multicolour printing press with rollers onto a pre-sealed core and the print is covered with a protective transparent wear layer that may be an overlay, a plastic foil or a lacquer. The production process is rather complicated and is only cost efficient in very large production volumes.
Direct printing technology may be replaced with digital printing technology that is much more flexible and small production volumes can be economically manufactured. The difference between these two methods is mainly the printing step where the printing rollers are replaced by a digital non-contact printing process.
Digital printing may also be used to print on a paper sheet that is used in conventional laminate production and laminated under heat and pressure. The printing may be made prior or after impregnation. Such printing prior to impregnation is complicated since paper may swell and shrink during the printing and impregnation step and small quantities are not cost efficient to impregnate. Printing after impregnation on a melamine impregnated paper is very difficult since pigments applied on a melamine surface float during the pressing step when the melamine resin is in a liquid state. Such problems may partly be solved with a method where a raw paper, preferably comprising a base colour, is applied and fixed to the core prior to printing and impregnated paper or melamine powder is applied under and/or over the raw paper such that the resins from the impregnated papers penetrate into the raw paper during the pressing step.
Laminate floors may also have a surface of paper foils or plastic foils and such foil materials may also be printed digitally. A protective wear resistant transparent layer that generally is a polyurethane lacquer is used to covers the printed décor.
b) Powder Based Floors (WFF)
Recently new “paper free” floor types have been developed with solid surfaces comprising a substantially homogenous powder mix of fibres, binders and wear resistant particles hereafter referred to as WFF (Wood Fibre Floor).
The powder mix may comprise aluminium oxide particles, melamine formaldehyde resins and wood fibres. In most applications decorative particles such as, for example, colour pigments are included in the mix. In general, all these materials are applied in dry form as a mixed powder on a HDF core and cured under heat and pressure to a 0.1-1.0 mm solid layer. The powder is, prior to pressing, stabilized with moisture and IR lamps such that it forms an upper skin layer similar to a paper layer and this prevents the powder from blowing away during pressing. Melamine formaldehyde powder and wood fibres may be replaced by thermoplastic particles.
Several advantages over known technology and especially over conventional laminate floorings may be obtained such as increased wear and impact resistance, deep embossing, increased production flexibility and lower costs. An embossing depth of 0.2-0.7 mm may easily be reached.
Powder technology is very suitable to produce a decorative surface layer, which is a copy of stone and ceramics. In the past it was more difficult to create designs such as, for example, wood decors. However, recently digital powder printing has been developed and it is possible to create very advanced designs of any type by injecting ink into the powder prior to pressing. Problems related to paper impregnation may be completely eliminated since no impregnation is required. The surface structure is made in the same way as for laminate flooring by a structured press plate, a steal belt or an embossed matrix paper that is pressed against the powder. A major advantage compared to the other digital printing technologies is that the powder provides a base colour and no protective layer is needed above the print since the ink may penetrate into the powder. The penetration is however rather limited since the ink drops will be bonded to the first particle that they hit, mainly the wood fibres. Increased wear resistance may be reached if several printed powder layers are applied on each other or if a powder overlay is used as a protective layer applied over the digital print.
c) Melamine Formaldehyde Resin.
A basic substance in Laminate and WFF floors is the thermosetting melamine formaldehyde resin that is used as a binder. Melamine resin or melamine formaldehyde resin (generally shortened to melamine) is a hard, thermosetting plastic material made from melamine and formaldehyde by polymerization. Such resin, hereafter referred to as melamine, comprises three basic stages. The stages, A-stage, B-stage, C-stage are described in Principles of Polymerization, George Odian, 3rd edition, which is hereby incorporated by reference, including particularly pages 122 to 123. The first uncured A-stage is obtained when melamine, formaldehyde and water is boiled to a liquid substance with a dry content of about 50%. The second semi-cured B-stage is obtained when the liquid resin is used to impregnate, for example, an overlay paper that after the application of the liquid resin is dried with heat. The molecules have started to cross link but the resin is still possible to cure in a final stage if the drying of the resin is made during a rather short time, for example, one minute and with a heat of about 90-120° C.
The B-stage may also be obtained by spraying the liquid resin over hot air such that the drops are dried and a dry semi-cured melamine formaldehyde powder is obtained that comprises small round spherical particles with a diameter of about 30-100 microns (0.03-0.10 mm).
The final completely cured C-stage is obtained when, for example, the melamine impregnated paper or the WFF powder is heated to about 160° C. under pressure during 10-20 seconds. The dry melamine formaldehyde resin becomes softer, melts and cures to a fixed form when the temperature increases during the pressing. The curing is dependent on temperature and heating time. Curing may be obtained at lower temperatures and longer time or at higher temperature during shorter time. Spray dried melamine powder may also be cured under high temperature.
d) Wood Floors.
Wood floors are produced in many different ways. Traditional solid wood floors have developed into engineered floors with wood layers applied on a core made of wood lamellas, HDF or plywood. The majority of such floors are delivered as pre-finished floors with a wood surface that is coated with several transparent layers in the factory. The coating may be made with UV cured polyurethane, oil or wax. Recently wood floorings have also been produced with a digitally printed pattern that improves the design of the wood grain structure in wood species that do not have a sufficient surface quality.
e) Ceramic Tiles
Ceramic tiles are one of the major materials used for flooring and wall coverings. The raw materials used to form tiles consist of clay minerals, feldspar and chemical additives required for the shaping process. One common method to produce ceramic tiles uses the following production steps. The raw materials are milled into powder and mixed. Sometimes, water is then added and the ingredients are wet milled. The water is removed using filter pressing followed by spray drying into powder form. The resulting powder is then dry pressed under a very high pressure (about 400 bars) to a tile body with a thickness of 6-8 mm. The tile body is further dried to remove remaining moisture and to stabilize the tile body to a solid homogenous material. Recently dry pressing of large and thin panels have been introduced. Dry granular material is pressed with very high pressure up to 400 bars and panels with a size of 1*2 m and more and with thicknesses down to a few mm may be produced in a cost efficient way. Such panels may be used for wall panels and worktops. The production time has been reduced from several days to less than an hour.
Such panels may be cut and shaped with production tolerances that are superior to the traditional methods and the may even be installed in a floating manner with mechanical locking systems. One or several layers of glaze, which is a glass like substance, are applied on the tile body by dry or wet methods. The thickness of the glazing is about 0.2-0.5 mm. There may be two glazes on the tile, first a non-transparent glaze on the tile body, then a transparent glaze on the surface. The purpose of tile glazing is to protect the tile. The glaze is available in many different colours and designs. Some glazes can create different textures. The tile is after glazing fired in a furnace or kiln at very high temperatures (1,300° C.). During firing, the glaze particles cures and melt into each other and form a wear resistant layer. Roller screens are often used to create a decorative pattern. The contact nature of the rotary screen-printing has many disadvantages such as breakages and long set-up times. Several tile producers have therefore recently replaced this conventional printing technology with digital ink jet printing technology that offers several advantages. Generally oil based inks are used and the print is applied on the pressed tile body or on a base glazing that is applied in wet form and dried prior to printing. A transparent glaze layer may be applied on the digital print in order to improve the wear resistance. Digital non-contact printing means no breakages and possibility to use thinner tile bodies. Short set-up times, randomized printing with no repetition effects and ability to print on surfaces of variable structures and on tiles with beveled edges are other major advantages.
Additional circumstances that have contributed to the introduction of the digital printing technology in the tile industry is the fact that ceramic tiles are rather small compared to, for example, laminate and powder based floors that are produced as large pressed boards of about 2.1*2.7 m. Rather small printers with limited number of print heads may be used in the tile industry and the initial investment is rather limited. Oil based inks have a very long drying time and clogging of nozzles may be avoided. Other advantages are related to the glazing that provides a base colour. Generally smaller amounts of pigments are required to form a tile pattern on a base colour than to provide an advanced wood grain design on a HDF or paper material used in laminate floorings where impregnation and lamination creates additional problems.
f) LVT Floorings.
Luxury Vinyl Tiles, generally referred to as LVT floorings, are constructed as a layered product. The name is somewhat misleading since a major part of LVT floors have a plank size with a wood pattern. The base layer is made primarily of several individual base layers comprising different mixtures of PVC powder and chalk filler in order to reduce material costs. The individual base layers are generally about 1 mm thick. The base layer has a thin high quality printed decorative PVC foil on the upper side. A transparent wear layer of vinyl with a thickness of 0.1-0.6 mm is generally applied on the decorative foil. Glass fibres are often used to improve thermal stability. The individual base layers, glass fibres, the decorative foil and the transparent layer are fused together with heat and pressure in continuous or discontinuous presses. The transparent layer may include a coating of polyurethane, which provides additional wear and stain resistance. Some producers have replaced the transparent vinyl layer with a polyurethane layer that is applied directly on the decorative foil. Recently new types of LVT floors have been developed with a base layer thickness of 3-6 mm and with edges comprising mechanical locking systems that allow floating installations. LVT floors offer several advantages over, for example, laminate floors such as deep embossing, flexibility, dimensional stability, moisture resistance and lower sound. Digital printing of LVT floors is only on an experimental stage but would, if introduced, provide major advantages over conventional printing technology.
As a summary it may be mentioned that digital printing is used in several floor types to create a décor. However the volumes are still very small, especially in wood and laminate flooring applications, mainly due to high cost of the ink and high investment cost for the industrial printers. The flexibility that the digital printing technology provides is limited by the embossing that is fixed and not possible to adapt to the variations of the digitally printed décor. It would be a major advantage if the ink cost could be reduced, if more cost efficient printing equipment could be used in an industrial scale, if a higher wear resistance could be reached without separate protective layers and if variations in the embossed structures may be formed that correspond to variations in the digitally printed pattern.
Definition of Some Terms
In the following text, the visible surface of the installed floor panel is called “front side”, while the opposite side of the floor panel, facing the sub floor, is called “rear side”.
By “up” is meant towards the front side and by “down” towards the rear side. By “vertically” is meant perpendicular to the surface and by “horizontally” parallel to the surface.
By “pigments” is meant a very fine powder of solid colorant particles.
By “pigment ink” is meant an ink comprising pigments that are suspended or dispersed throughout a carrier fluid.
By “binder” is meant a substance that connects or contributes to connect two particles or materials. A binder may be liquid, powder based, a thermosetting or thermoplastic resin and similar. A binder may consist of two components that react when in contact with each other. One of the components may be liquid and the other dry.
By “matrix” also called “mat” is meant a material that forms an embossed surface structure when the material is pressed against a surface.
By “Embossed In Register” or EIR means that a printed décor is in register with an embossed structure.
By “digital ink jet printing” is meant a digitally controlled ejection of drops of fluid comprising a colorant from a print head onto a surface.
By “digital print” is meant a digitally controlled method to position colorant onto a surface.
By “colourant” is meant any material (dye, organic or inorganic pigments, small coloured particles of any material etc.) that may be used to provide a colour on a surface preferably due to selective absorption or reflection of different wavelengths of light.
By “panel” is meant a sheet shaped material with a length and width that is larger than the thickness. This rather broad definition covers, for example, laminate and wood floors, tiles, LVT, sheet shaped wall coverings and furniture components.
Known Technique and Problems Thereof
The generally known technologies, which may be used to provide a digital print and an embossed surface structure, are described below. The methods may be used partly or completely in various combinations with preferred embodiments of the invention in order to create a digital print or a digital embossing according to this disclosure of the invention.
High definition digital ink jet printers use a non-impact digital printing process. The printer has print heads that “fire” drops of ink from the print head to the surface in a very precise manner.
Multipass Printing, also called scanning printing, is a printing method where the printer head moves transverse above the surface many times to generate an image. Such printers are slow but one small print head can generate a bigger image.
Industrial printers are generally based on a Single Pass Printing method, which uses fixed printer heads, with a width that corresponds to the width of the printed media. The printed surface moves under the heads. Such printers have a high capacity and they are equipped with fixed print heads that are aligned one after each other in the feeding direction. In general each head prints one colour. Such printers may be custom made for each application.
FIG. 1a, shows a side view of an industrial single pass digital ink jet printer 35 comprising five digital print heads 30a-e, which are connected with ink pipes 32 to ink containers 31 that are filled with ink of different colours. The ink heads are connected with digital data cables 33 to a digital control unit 34 that controls the application of the ink drops and the speed of the conveyor 21 that must be able to displace the panel under the print heads with high precision in order to guarantee a high quality image comprising several colours.
FIG. 1b shows a top view of a wood grain print P provided on a panel surface 2. The surface of a floor panel is often embossed with a basic structure 17 that is the same for several basic decors as shown in FIG. 1c. Advanced floors use a so-called EIR (Embossed In Register) embossing 17 that is coordinated with the printed pattern P as shown in FIG. 1d. 
A normal width of an industrial print head is about 6 cm and any lengths may be printed. Wide areas of 1-2 m may be printed with digital printers comprising several rows of print heads aligned side by side. 166 print heads may be needed to provide a 5-colour print on a 2 m wide laminate floor panel and the print may be destroyed if only a few nozzles in one print head are blocked by dry ink.
Number of dots per inch or DPI is used to define the resolution and the printing quality of a digital printer. 300 DPI is generally sufficient to, for example, print wood grains structures of the same quality presently used in conventional laminate floorings. Industrial printers can print patterns with a resolution of 300-600 DPI and even more and with a speed exceeding 60 m/min.
The print may be a “full print.” This means that the visual printed décor is mainly created by the ink pixels applied on the surface. The colour of a powder layer or a base colour of a paper has in such an embodiment, in general, a limited effect on the visible pattern or décor.
The print may also be a “part print”. The colour of another underlying layer is one of the colours that are visible in the final décor. The area covered by printed pixels and the amount of ink that is used may be reduced and cost savings may be obtained due to lower use of ink and increased printing capacity compared to a full print design. However a part print is not as flexible as a full print since the base colours are more difficult to change than when a full print is used.
The print may be based on the CMYK colour principle where the white colour is provided by the surface. This is a 4-color setup comprising cyan, magenta, yellow and black. Mixing these together will give a colour space/gamut, which is relatively small. To increase specific colour or the total gamut spot colours may be added. A spot colour may be any colour. The colours are mixed and controlled by a combination of software and hardware (print engine/print heads). The flexibility may also be increased considerably by adding a white colour to the printer.
New technology has been developed by CeraLoc Innovation Belgium BVBA, a subsidiary of Välinge International AB that makes it possible to inject a digital liquid print into a powder layer. This new type of “Digital Injection Print” or DIP is obtained due to the fact that printing is made into a powder that is cured after printing. The ink and the print are embedded into the cured layer and they are not applied on a layer as when conventional printing methods are used. The print may be positioned in several dimensions horizontally and vertically in different depths. This may be used to create 3D effects when, for example, transparent and preferably bleached wood fibres are used. A two-layer print may also be used to increase the wear resistance. No protective layers of, for example, overlay are needed that disturb the original design with grey shadings.
The DIP method may be used in all powder based materials, which may be cured after printing. However, the DIP method is especially suitable to be used when the powder comprises a mix of wood fibres, small hard wear resistant particles and a melamine resin. The surface layer may also comprise thermoplastic material, for example, vinyl particles, which are applied in powder form on a surface. This allows that the print may be injected in the vinyl powder particles. An improved design and increased wear resistance may be reached even in such materials.
A suitable printer head has to be used in order to obtain a high printing quality and speed in powder based layers and other layers as described above. A printer head has several small nozzles that can shoot and apply droplets of inks in a controlled way.
Industrial inkjet systems, are broadly classified as either continuous inkjet (CIJ) or drop on demand (DOD) systems.
CIJ ejects drops continuously from the print head. The drops pass through a set of electrodes, which impart a charge onto each drop. The charged drops then pass a deflection plate which uses an electrostatic field to select drops that are to be printed and drops to be collected and returned for re-use.
DOD ejects drops from the print head only when required and all drops are applied on the surface.
CIJ is primarily used for coding and marking of products. DOD inkjet technology is currently used in most existing industrial inkjet applications where a high quality décor is required.
A normal size of an ink droplet is about 2-4 picoliters (=1*10−12 litre or 0.000001 mm3). The size of each droplet may vary, dependent on ink type and head type, normally between 1-40 picoliters and this corresponds to a droplet that has a diameter of about 10-30 microns. Smaller droplets enable high-resolution images. Some printer heads can shoot different droplet sizes and they are able to print a grey scale. Other heads can only shoot one fixed droplet size. It is possible to design print heads that may fire bigger drops up to 100-200 picoliters or more.
Several technologies may be used to shoot the drops out of the nozzles.
Thermal print head technology generally referred to as bubble jet printing, use print cartridges with a series of tiny chambers each containing a heater. To eject a droplet from each chamber, a pulse of current is passed through the heating element causing a rapid vaporization of the ink in the chamber to form a bubble, which causes a large pressure increase, propelling a droplet of ink out through the nozzle and to the surface. Most consumer inkjet printers use thermal printer heads. Such thermal printers are generally designed to apply water based inks with a viscosity of 2-5 centipoise (cps)
Recently large-scale thermal print heads with a printable width of 223 mm and with a printing speed of about 20 m/min or more have been developed by Memjet. The print head contains 5 ink channels and two rows of nozzles per channel. Each individual nozzle structure is about 30-microns across, enabling 800 dpi, with the second row of nozzles for each colour slightly offset from the first to deliver 1600 dpi in combination. A Memjet print head can continuously fire up to 750 million 2 picoliters drops with a 14 micron drop diameter per second. The print head cost is less than 10% of the costs for conventional Piezo heads with similar capacity. Such thermal printers may apply water based substances with a viscosity of 0.7-1.5 centipoise which is similar to water viscosity (1 centipoise at 20° C.). The Memjet print head comprises a self-cooling system with the heating element in the middle of the ink chamber. As drops are ejected, new ink flows into the chamber and cools the heating element.
Thermal technology imposes the limitation that the ink must be heat-resistant, generally up to 300° C. because the firing process is heat-based. This makes it very difficult to produce pigment based multi colour thermal heads. The Memjet print heads are designed for dye based ink and are therefore not used in the flooring industry and in industrial applications where high quality pigment based inks are required.
Most commercial and industrial inkjet printers and some consumer printers use the piezoelectric printer head technology, which is the major technology used in the flooring industry. A piezoelectric crystal material (generally called Piezo) in an ink-filled chamber behind each nozzle is used instead of a heating element. When a voltage is applied, the piezoelectric material changes shape, which generates a pressure pulse in the fluid forcing a droplet of ink from the nozzle. A Piezo print head configuration may use different basic deformation principles to eject drops from a nozzle. These principles are generally classified in squeeze, bend, push and shear print head technologies. A piezoelectric crystal may also be used to create acoustic waves as it vibrates and to cause the ink to break into droplets at regular intervals. Piezo inkjet allows a wider variety of inks and higher viscosity than thermal inkjet. The ink has generally a viscosity in the range of 2-12 centipoise and is very suitable to apply pigment based ink. In industrial applications print heads that may handle high viscosity inks are often used since the initial viscosity of the ink decreases considerably during production when temperature may increase to 40° C. or more and a low initial viscosity may fall below the minimum level that is required for a proper functioning of the print head.
FIG. 1e shows how ink drops 56 are ejected according to the bend mode of piezoelectric material. A Piezo print head 30 comprises arrays of very small holes generally called jets 50 from which droplets 56 of ink 58, with pigments 12, are ejected on a paper surface.
The ink 58 flows from an ink container via an ink inlet 55 into an ink chamber 52. Electrical pulses bend a Piezo crystal 51 and a membrane 53. This deformation creates a pressure pulse that ejects an ink drop 56 from the nozzle 54. Different drop sizes may be formed by varying the electrical charge. The nozzles are typically about 10 microns in diameter. Typical drop volumes are in the range of 2-5 picoliters producing printed ink spot sizes 57 on a surface in the range of 10-20 microns. Each droplet may contain about 20% pigments. The remaining part is a liquid carrier and resins needed to connect the pigments to the surface.
A digital image contains a grid of a fixed number of rows and columns of pixels, which are the smallest individual element in a digital image. The grid is called a raster. The pixels, which represent images as a computer file, are of a uniform size and shape. They do not overlap and they touch adjacent pixels on all sides. Raster images can be created by a variety of input devices, for example, a digital camera. All known printers use a Raster Image Processing (RIP) software, which takes an image file input and produces a colour profiled, screened, bitmap output that controls the print heads and provides the necessary data that is needed to apply an ink drop on a surface in a pre-determined raster pattern R1-R4 as shown in FIG. 1e. 
A lot of ink types may be used. The main components are colourants that provide the colour, a binder that bonds the colourants to the surface and a liquid carrier that transfers the colorant and the binder from the print head in well-defined small drops to a surface with a non-contact application method. The colourant is either a dye or pigment or a combination of both. The carrier fluid may be water-based or solvent based. The carrier fluid evaporates and leaves the colourant on the surface. UV curable inks are similar to solvent based inks but the carrier fluid cures when exposed to strong UV light.
A main problem for all types of inks and print heads is that when ink dries by evaporation it may dry up and clog the nozzles. Industrial printers may be equipped with an ink circulation system that circulates the ink through the jets in order to increase the so called “decap” time which is the amount of time a print head can be left uncapped and idle and still fire ink drops properly. A short decap time or clogging may results in permanent nozzle loss and undesired lines may be formed over the whole surface when single pass printers are used. Especially pigment-based inks comprising polymer binders systems have a tendency to dry out and it would be a major advantage if the decap time could be increased and nozzle clogging could be avoided.
A dye is a colourant that is dissolved fully into the carrier fluid and the ink is a true solution.
Pigments are very fine powder of solid colourant particles that are suspended or dispersed throughout a liquid carrier. Pigment based inks are generally individually mixed together by using colour pigments and several chemicals. Pigments used in digital ink are very small and have an average particle size of about 0.1 micron. The common size of the nozzles is about 10-20 microns, which means that the pigment particles have enough space to pass through the nozzle channels in the print head. The nozzles may still be blocked by the ink itself and pigments that form clusters of particles. High quality pigment ink should keep the pigment suspended in the carrier fluid for a long period of time. This is difficult particularly at the rather low viscosities, which are required for a good functioning of the print heads. Pigments have a natural tendency to settle out and fall down in the liquid carrier. In high quality pigment ink, no settling out of the pigment should normally occur. Advanced ink circulation systems are used to avoid such problems related to ink with high pigment content.
Pigment inks are generally more light stable, especially when exposed to UV light, and more fade resistant than dye-based inks. They are therefore used in almost all flooring applications. Water based digital inks comprising colour pigments are especially suitable for flooring applications and may provide a high quality printing method in many different materials.
Generally the pigments do not stick to a smooth surface. They are similar to sand particles and may be easily removed from most dry and smooth surfaces. The water based carrier fluid is therefore generally mixed with small amounts of several other additives to provide special ink and print properties such as binders that provide the adhesion of the pigments to a surface, dot gain, pH level, drop formation, corrosion of the print head, fade resistance, etc. The inclusion of resins that serve as binder in the ink composition limits the possible amount of pigments, as both components increases the ink viscosity.
Colour pigments as raw materials are rather cost competitive especially as rather large particles of about one micron but the production of pigment based inks comprising very small particles and other inks for digital printers is very complicated and expensive and this results in a very high cost for the ink that normally may be in the region of about 50-100 EUR/litre. About 50-100 m2 of flooring may be printed with one litre (20-10 g/m2) if a full high quality print is applied and this gives a printing cost of 1-2 EUR/m2. The costs for a conventional printed floor surfaces where printing cylinders are used are only 10% of the cost for digitally printed floor surfaces. This means that digital printing based on conventional pigment based liquid ink is only cost competitive in small series when very high production flexibility is required.
Digital ink jet printers use a non-contact method to apply the ink on a surface. Laser printing however is based on a contact method where a laser beam projects an image on an electrically charged rotating drum, generally called photo conductor drum. Dry ink particles, generally called toner, are then electrostatically picked up by the drum's charged areas. The ink comprises fine and very well defined spherical particles of dry plastic powder such as, for example, styrene acrylate copolymer or polyester resin which is mixed with carbon black or colouring agents. The particles have a diameter of about 8-10 microns when 600 DPI printing resolution is required. Some laser printers use even smaller particles with a diameter of about 5 microns. The thermosetting plastic material acts as a binder. The drum prints the image on a paper by direct contact and heat, which fuses the ink to the paper by bonding the plastic powder to the paper. Colour laser printers use the CMYK principle with coloured dry ink, typically cyan, magenta, yellow, and black that are mixed in order to provide a high quality coloured image.
The laser technology with the impact method is not used for printing of a flat panel surfaces such as a floor panel surfaces.
3D printing is a well-known technology that is used to apply and connect several layers of liquid substance, powder or foils on each other in order to create advanced three dimensional structures. The technology is mainly used for prototype production of small complex products. Several hundred layers may be applied on each other. Several principles are used to build layered structures. According to one main principle powder layers are applied on each other and some parts are bonded by a liquid UV cured substance applied by a digital print head on each powder layer. Non-bonded powder is removed when the whole structure of the product is formed. Another principle uses a small glue gun that applies several layers of hot liquid plastic material in several layers. 3D printers have a very low productivity and construction of even small objects can take several hours. 3D printers are not used to create flat décors on a surface where colourants are applied side by side and where non-bonded powder must be removed after each application of a layer. The structure of the layers applied on each other will be destroyed if pressing is used to cure the layers.
Dye-sublimation printers use a long roll of transparent film of red, blue, yellow, and grey coloured cellophane sheets that are attached together end to end. Embedded in this film of many sheets attached to each other are solid dyes corresponding to the four basic colours cyan, magenta, yellow and black and each sheet comprises only one colour. The “print head” contains thousands of small heating elements that produce varying amounts of heat and the dye is transferred to a coated paper with “sublimation” which means that the dye when heated turns into a gas without first turning into a liquid. Such thermal print heads, hereafter referred to as heating print heads in order to differentiate such heads against the thermal print heads used in bubble jet printing, heats up as it passes over the film, causing the dyes to vaporize before they return to solid form on the paper. This method eliminates the used of liquid ink and may provide a high photo quality with dyes that are transparent and that blend into a continuous-tone colour. However, the method has many disadvantages. Each sheet must have the same size as the printed surface and the whole sheet is used even if a small part of the surface is printed with a specific colour. In order to eliminate some of the disadvantages dye sublimation heat transfer imprinting printers have been developed, which use special inks comprising sublimation particles. A conventional inkjet printer may be used to print an image with such sublimation ink on a special paper or foil. The image is thereafter transferred by pressure and heat to a polyester material or a surface that has a polymer coating.
Thermal printing with heating print heads are also used to create digital prints directly on a heat sensitive paper or indirectly with a thermal transfer printing method where the heat is applied on a heat sensitive transfer film. These printing methods are mainly used to apply one colour on a paper and to print, for example, labels. The heating print heads have several advantages. They are reliable since there are no risks for clogging of inks and the price is cost competitive. The major disadvantages are related to high cost for the paper or transfer film and the colour limitations to mainly one colour. Heating print heads are available in widths of up to 200 mm and may provide a resolution of up to 600 DPI.
Digital printing is a very flexible method that may provide a high quality print but it cannot be fully utilized in industrial application and especially not in floorings due to the high cost for the ink, problems related to drying and clogging of nozzles, especially when pigment based inks are used and the need for special protective layers that are costly and not completely transparent. The high ink costs are primarily caused by the need to mill down the colour pigments to well-defined very small particles and to disperse the particles throughout the carrier fluid. It would be a major advantage if digital images may be created with colour pigments that may be larger, that are not dispersed in a carrier fluid and that are not applied as drops by small nozzles. It would also be a major advantage if digital images may be formed with higher wear resistance and without protective layers.
The majority of all the above-described floors and especially digitally printed floors have an embossed surface structure, especially when the decorative printed décor is a wood pattern. The embossed structure was in the past provided as a separate general structure that was used for many different décor types. Recently most floor producers have introduced the so-called Embossed In Register EIR method where the embossed surface structure is specifically formed for each type of wood species and the embossing is made in register with the printed décor. This provides advanced designs that are difficult to differential from the natural materials such as wood and stone. The embossing is obtained when the surface is pressed against a structured matrix that may be a steal plate, steal belt, metal roller, plastic foil or coated paper. The décor must be positioned with high precision against the pressing matrix. Generally digital cameras and mechanical devices that adjust the final position of the panel such that it matches the décor before pressing are used to obtain such positioning. One specific problem related to laminate flooring is the fact that the printed-paper swells and shrinks in an uncontrolled way during impregnation and the size of the décor may vary between different impregnate paper sheets.
The flexibility of digital printing is also limited in connection with EIR surfaces since the printed décor must always be adapted to the embossed matrix. A common feature for all such floors as described above is that all surfaces in a production batch have the same basic structure and are not possible to adjust and adapt to any changes in the décor. This repetition effect of the embossed structure provides a floor surface that is not similar to a wood floor where practically all panels have different designs and structures due to the wood grain structure of the wood. Copies of stone and other natural materials cannot be produced in a way that is a true copy of the natural material where design and structure generally is perfectly combined and all panels are different.
The digital ink jet technology is mainly used to obtain advantages related to the possibility to create a high-resolution image in a flexible way. However, the other aspects of the technology, mainly related to the possibility to apply a liquid substance very precisely with a non-impact method, have not been fully utilized or developed, especially not in applications where a décor is applied on a large size panel comprising a surface that during production and especially after printing receives it final shape and properties in production steps comprising high pressure and heat.
It is known that powder applied on a liquid substance could be used to create raised portions or an image on mainly a paper substrate and that the liquid substance may be applied digitally by ink jet. 3-D printing comprising several powder layers that are locally connected with a digital device such as an ink head and where excessive non-connected powder particles are removed in a final step is a well-known technology that may be used to create an embossed structure on a panel. It is also known that powder particles may be applied directly with a non-contact method on a surface comprising a binder or indirectly with a contact method where a transfer method is used. Even combinations are known where a non-contact transfer method is used and the powder is detached from the transfer surface with heat or scraping.
U.S. Pat. No. 3,083,116 describes raised printing powder and a raised printing process comprising dusting a powdered resin upon a newly printed sheet, removing therefrom the excess powder which do not adhere to the wet ink, and applying heat to the powder retained on the sheet to fuse it so that particles thereof will flow together and adhere to the sheet. The powder may comprise a phenolic resin such as phenol, urea and melamine.
U.S. Pat. No. 3,440,076 describes a method of forming raised hard printed characters on a sheet of paper. An ink composition is printed on the paper and then contacted with a dry material. One of the ink composition and dry material contains a thermosetting resin and the other material a blowing agent and a curing agent. The dry powder material not adhering to the ink is removed and the resin associated with the printed character is then cured with heat at temperatures sufficient to fuse the powder.
U.S. Pat. No. 3,446,184 describes a method to form a sticky image copy. Toner powder is applied on a liquid forming and a portion of the powder is retained by the liquid coating, forming a visible image. Loose powder is removed and the sheet passes a heating unit where the retained powder is fused to form a permanent image.
U.S. Pat. No. 4,312,268 describes a method by which a water-based ink is applied digitally to a continuous web and fusible single colour powder material is applied to the web and on the ink. Some of the powder material is bonded to the liquid, and non-bonded powder material is removed from the web prior to heating of the web to dry the liquid and to fuse the powder material to the web by melting the powder. It is mentioned that the powder material may have a particle size in the range of 5 to 1000 microns and may have a melting point or fusing point in the range of 50 to 300° C. The powder material may be produced by dissolving or dispersing, respectively, a dye or a pigment in a resin or resin formulation, followed by grinding, spray chilling or the like to reduce the material to a fine powder. The powder material may provide abrasion resistant qualities to the ink that may contain phenolic resin. The liquid material, which is applied through the jets, may be clear and colourless water.
GB 2 128 898 describes a method to form raised decorative portions in a plastic tile. A decorative floor covering in tile form has a design printed on its upper surface. Particles such as inorganic sand particles are positioned on the upper surface of a plastic tile with at least some of the particles being placed on the tile surface in register with the design printed on the tile surface. Excess sand particles are removed. A cured wear layer overlies both the raised particle and the plastic base, whereby the wear layer surface in the areas containing particles and in the areas not containing particles will be of different gloss characteristics. The process requires the sprinkling of particles over an adhesive coated surface to retain the particles in registration with a printed design on the tile surface.
U.S. Pat. No. 6,387,457 describes a method of using dry pigments for printing applications related to automobile painting, security printing, general painting and cosmetics. A binder material is applied to a surface of a substrate uniformly or in a pattern. The binder is applied by ink jet, spraying, screen, off-set or gravure printing. Dry pigment is applied to the binder material in a pattern or uniformly. The dry pigment material comprises flakes of non-metallic material having a particle size less than about 100 micron. The flakes are aligned in a direction parallel with the surface of the substrate and a protective coating may be applied on the flakes.
EP 0 403 264 A2 describes a transfer method to form a multi-colour image on a drum that transfers the image to a paper. A fluid digital latent image is subsequently developed at a development station where coloured powder is applied to the fluent latent image and fixed to produce a visible and permanent image. Several digital print heads may be used that print with dyeless fluids comprising a mixture of water with polyhydric alcohols and their sub-sets of ethylene glycol, glycerol, diethylene glycol and polyethylene glycol. A powder toner is applied across the surface of the paper and a voltage is applied during this development. The voltage is then reversed to remove the toner from the background areas. Fixing is achieved by means of conventional copier fusing methods.
U.S. Pat. No. 5,627,578 describes a method to produce raised lettering and graphics in desktop printing applications by using thermographic powder and an ink jet printer to apply a liquid binder. The method is similar to the above described methods to produce raised text.
EP 0 657 309 A1 describes a multicolour transfer method utilizing a transfer paper carrying a pattern formed by ink jet and powder similar to the above described methods. The transfer method is intended for decorating ceramics.
WO 2007/096746 relates to systems and apparatuses for transferring granular material with a non-contact or contact method to a surface to be decorated, particularly for obtaining decorations on ceramic tiles. A liquid digital pattern is provided by ink jet on a transferring surface that may be a drum or a belt. The granular material is applied and bonded to the transferring surface and only bonded granulate material is moved to a transferring zone where heat is applied on one specific portion of the transferring surface in the transferring zone in order to detach the granular material from the transfer surface and to apply the granular material on the receiving surface. The granules may also be detached by scraping. The major advantage with this method is that only particles that form the final image are applied on the receiving surface. The major disadvantage is that heating must be sudden and the particles must be released from the transferring zone and they must fall down on the receiving surface in a very controlled way in order to obtain a high resolution image. High resolution can only be obtained with rather heavy particles that fall by gravity on the receiving surface. The granular material used in the invention is of the type comprising non-porous granules, such as, for example, grits of vitreous materials or sintered mixtures, sands etc. in the various ranges of granulometry from 30 μm to 800 μm, preferably ranging from 50 μm to 150 μm. A transfer print with a contact method is also described.
WO 2011/107610 describes a method to create an elevation or an embossing on a floor panel in order to avoid the use of expensive press plates. The method is the same as the known methods to create a raised print. It describes a method to produce a floorboard by printing a curable substance for creating an elevation on the panel. The elevation may be applied on a basic decorative pattern that is directly printed or laminated on the panel. The curable substance may comprise wear resistant particles. The curable substance may be digitally printed on the panel by first printing a liquid in a pre-defined pattern and then providing an intermediate substance that may comprise a powder. The curable substance may be cured by UV radiation or may be a varnish.
EP 2 213 476 A1 describes that a pre-determined pattern may be digitally printed on a carrier with curable liquid so as to form an embossing decoration pattern, which is pressed on the overlay. The curable liquid may be a plastic, which becomes rather rigid after curing, for example, a plastic containing ink. This method is not suitable for floor applications. The digital printing head can only print a very thin layer with a thickness of about 10-20 micron. Thicknesses of at least 100-200 micron that are required to form an embossing in laminate and 200-700 micron to match the requirement of powder based floors are not possible to produce in an economic way.
WO 2012007230 describes a method to form a 3-D structure on a furniture or floor panel with a digitally controllable device. A décor is applied with a flat three-dimensional structure of powder based coating material comprising one or more layers, which are locally solidified by a digitally controllable device under the action of light and or heat radiation. Excess non-solidified coating material is removed in a final production step. The three dimensional structure may be digitally printed. A liquid coating material is applied on the 3-D structure as a protective layer.
The majority of the known methods are based on direct application of powder on a surface comprising a binder pattern. They are mainly used to create raised text or three-dimensional decors, which are cured and protected by a liquid coating. Such methods are not suitable for flooring application where the surface is cured and an embossed structure is formed with heat and pressure applied on the surface.
The above description of various known aspects is the applicants' characterization of such, and is not an admission that the above description is prior art when the described products, methods and equipment are used partly or completely in various combinations.